Age variations in the properties of human tibial trabecular bone and cartilage

Initiated and motivated by clinical and scientific problems such as age-related bone fracture, prosthetic loosening, bone remodeling, and degenerative bone diseases, much significant research on the properties of trabecular bone has been carried out over the last two decades. This work has mainly focused on the central vertebral trabecular bone, while little is known about age-related changes in the properties of human peripheral (tibial) trabecular bone. Knowledge of the properties of peripheral (tibial) trabecular bone is of major importance for the understanding of degenerative diseases such as osteoarthrosis and osteoporosis, and for the design, fixation and durability of total joint prosthesis. The specific aims of the present studies were: 1) to investigate normal age-related variations in the mechanical, physical/compositional, and structural properties of human tibial trabecular bone; and 2) to investigate the age-related and osteoarthrosis-related changes in the mechanical properties of the human tibial cartilage-bone complex; and 3) to evaluate mutual associations among various properties. Normal specimens from human autopsy proximal tibiae were used for investigation of age variations in the properties of trabecular bone and the cartilage-bone complex, and osteoarthrotic specimens were used for the investigation of changes in the mechanical properties of the cartilage-bone complex induced by this disease process. The mechanical properties and physical/compositional properties of trabecular bone were quantified by means of standard techniques, and trabecular bone structure was quantified by means of unbiased three-dimensional methods. The present study demonstrated that the mechanical properties, such as Young's modulus, ultimate stress, ultimate strain and failure energy, and the densities, such as apparent, apparent ash and collagen densities of human tibial trabecular bone have significant relationships with age. Tissue density and mineral concentration remain constant throughout life. Trabecular bone is tougher in the younger age, i.e. fracture requires more energy. Collagen density was the single best predictor of failure energy, and collagen concentration was the only predictor of ultimate strain. The decrease in mechanical properties of trabecular bone mainly is a consequence of the loss of trabecular bone substance. This study showed that the degree of anisotropy (preferential orientation of trabeculae), mean marrow space volume, and bone surface-to-volume ratio increased significantly with age. Bone volume fraction, mean trabecular volume, and bone surface density decreased significantly with age. Connectivity did not have a general relationship with age, yet a trend exists. Age-related changes in the microstructural properties had the same trends for both medial and lateral condyles of the tibia. The observed increase of anisotropy may be interpreted as the consequence of structural adaptation secondary to age-induced bone loss. The aging trabeculae align more strongly to the primary direction, which is parallel to the longitudinal loading axis of the tibia. The mechanical properties of the normal cartilage and bone vary with age and respond simultaneously to mechanical loading. Both cartilage and bone in early-stage OA are mechanically inferior to normal, and OA cartilage and bone have lost their unit function to mechanical loading.